The present invention relates to a machine for manufacturing extruded tubes made of synthetic and non-synthetic material, particularly for small-diameter polyvinylchloride tubes, for example of the kind used in medical applications for providing dialysis circuits and others.
Machines for the production of polyvinylchloride tubes intended for the most disparate types of use are known which substantially comprise an extrusion head inside which a passage for the semi-liquid or pasty synthetic material is defined which opens outward through a gauged hole. A male element of the head is arranged inside the passage for the synthetic material and extends coaxially in said passage until it is proximate to the gauged exit hole. A channel is defined in the extrusion head and in the related male element, is fed with air at a preset pressure and is open outward at the end of the male element which is arranged proximate to the gauged exit hole. At the beginning of extrusion, in the case of low-thickness tubes, the first portion of the tube extruded through the gauged exit hole is closed, for example by pressing, so that during extrusion the inside of the tube is pressurized by the air fed into said channel; the pressure of said air is determined according to the dimensions of the tube to be obtained.
In the case of tubes with higher thickness, the pressurization operation becomes unnecessary.
At the exit from the extrusion head, the tube is inserted in a channel in which a cooling liquid, for example water, is contained, and is subsequently engaged by entraining means which exert a traction on the tube which is proportional in extent to the outer diameter of the tube to be obtained. In practice, by keeping the air pressure constant inside the tube, if it is pressurized, it is possible to vary its internal and external diameter according to the requirements simply by varying its entrainment speed.
In the manufacture of polyvinylchloride tubes which are intended to engage coupling sleeves in which the seal between the sleeve and the tube is provided on the outer surface of the tube, the outer diameter of said tube and even more particularly its external circumference must be provided with extreme precision.
Due to this requirement, machines intended for the production of these kinds of tubes are equipped with checking devices which are interposed between the extrusion head and the entrainment means and constantly check the external diameter of the tube. Such checking devices are generally constituted by optical gauges which sense the diameter of the tube by emitting a light beam in a direction which is perpendicular to its axis or which, as in the case of laser light devices, measure two diameters of the tube which are arranged substantially perpendicular to one another.
Such known types of checking devices, while allowing a satisfactory measurement of the diameter of the tube in terms of dimensions, have some disadvantages.
An optical measurement of the diameter of the tube is in fact capable of sensing possible surface irregularities of the tube only when such surface irregularities affect the diametrically opposite regions of the surface at which the measurement is performed. When said surface irregularities affect regions of the tube which are not affected by the measurement, these irregularities are not sensed. Even in the case of checking devices which perform a double measurement, sensing two mutually perpendicular diameters, the possible presence of surface irregularities on the regions not affected by these measurements is not sensed by the device.
The uniformity of the outer surface of the tube is extremely important, by virtue of the fact that the seal between the tube and the sleeves is generally obtained by gluing on the outer surface of the tube. Due to this fact, the correct coupling between the sleeve and the tube can be compromised, not only when the dimensions of the tube are outside a preset tolerance range but also when irregularities occur on the outer surface of the tube though dimensional tolerances are met. Besides, as regards the correct coupling between the tube and the sleeve, the possible oval configuration of the tube is scarcely important, since the tube is made of elastically flexible material and is therefore capable of assuming a correct cylindrical configuration once it is inserted in the sleeve.
Another disadvantage which can be observed in optical control devices is related to their cost, as well as to the great precision required in adjusting these devices.